Advances have since been in higher strength with respect to automobile steel-components (use of a high tensile-strength steel) for attaining compatibility of safety at a collision and weight reduction. Meanwhile, at the time of cold stamping of the high tensile-strength steel, there exists a problem such as an increase in forming load, deterioration in size accuracy, and so forth.
There has been known a hot-press forming technology as a means for solving the problem, the technology being for use in the press forming of a steel sheet as a material, in as-heated state, to thereby concurrently realize the forming and the higher strength. With this method, a steel sheet in high-temperature state is held at the bottom dead center of forming to be cooled, while being formed by use of a tool (a punch, and a die) to thereby effect rapid cooling by dissipation of heat from the steel sheet into the tool, and quenching of the material is executed. A formed product (a steel component, a steel member) excellent in size accuracy, and high in strength can be obtained by virtue of this hot-press forming technology.
Further, with an automobile steel component, a steel sheet with zinc coating applied thereto is often used from the viewpoint of corrosion resistance. However, in the case where the hot-press forming is applied to a galvanized steel sheet, intergranular cracking occurs to the formed product, due to liquid metal embrittlement (hereinafter referred to simply as LME on occasions), and the cracking poses a major problem in practical usage.
As a means for solving LME, there has been known a technology whereby both the coating and the composition of a material are optimized, and a galvanized steel sheet is quenched before the hot press forming is executed, as described in, for example, patent literature 1.